https://www.gamespot.com/articles/pokemon-go-mewtwo-coming-to-regular-raid-battles-f/1100-6461673/

Pokemon Go developer Niantic recently gave players another chance to catch Moltres as a reward for completing the game's Global Challenge, but that isn't the only special encounter fans have unlocked in the game. Niantic has also announced the Ultra Bonus Event, which kicks off this week and will bring four Legendary Pokemon from the Kanto region to Raid Battles for a limited time.

First, beginning September 13, the three Legendary birds Articuno, Zapdos, and Moltres will return to Raid Battles around the world until September 20. Not only does this give you another chance to catch any of the three you may have missed, you'll also have a chance to encounter their Shiny versions during the event. Additionally, other Pokemon originally from the Kanto region will appear much more frequently in the wild and in Raid Battles until September 30.